Ran across this excellent article with understandable text and clear color photos. In addition to the great articles on the BeamAlarm site, felt most on the forum would benefit from this as well.
Servicing Aqua-Hot Heating Systems (http://www.rvtechmag.com/tech/38_aquahotsvc.php)
If you are near the Houston area, Rudy will walk you through the service. He will come out and supervise while he teaches you to service your unit. Gotta Love Rudy! His cost is very reasonable.